1. Technical Field
The present invention relates to a nitride semiconductor light-emitting device and a method of manufacturing the same.
2. Description of the Related Art
A nitride semiconductor light-emitting device having a one-side two-electrode structure obtained by stacking an n-type nitride semiconductor layer, a light emitting layer, a p-type nitride semiconductor layer and the like on an insulating sapphire substrate and arranging a p-side electrode and an n-side electrode on such a multilayer structure is known in general. In the nitride semiconductor light-emitting device having such a one-side two-electrode structure, however, the electrodes are not formed on symmetrical positions of the upper and lower surfaces of the chip, and hence luminous intensity is not uniformized in plane, but emission concentrates on the p-side electrode or the n-side electrode. Further, it is difficult to increase the size of the chip and the chip is easily deteriorated by aging due to the aforementioned problem. In addition, the pad electrodes provided on one surface occupy large ratios in the surface area of the chip, and it is also difficult to reduce the size of the chip.
In order to solve the aforementioned problems, Japanese Patent Laying-Open No. 2001-244503, for example, proposes a method of manufacturing a nitride semiconductor light-emitting device including the steps of successively growing an n layer made of a gallium nitride-based semiconductor, a gallium nitride-based semiconductor active layer and a p layer made of a gallium nitride-based semiconductor on a substrate, successively forming a p-side ohmic electrode such as an Ni—Pt electrode, for example, and a first conductive adhesive layer made of Au—Sn on the p layer, bonding a conductive substrate prepared by successively forming a layer made of Au and a second conductive adhesive layer made of Au—Sn to the above-mentioned substrate by bonding the second conductive adhesive layer and the first conductive adhesive layer to each other and separating the substrate and a nitride semiconductor light-emitting device having a vertical electrode structure obtained by this method. In the nitride semiconductor light-emitting device described in this document, however, light extraction efficiency is inferior due to low reflectance of the electrode.
A structure employing a distributed Bragg reflector (DBR) is known as a device structure for improving light extraction efficiency. For example, National Patent Publication Gazette No. 2007-536725 discloses a method of manufacturing a semiconductor device by forming an n-type GaN layer, a multiple quantum well (MQW) layer and a p-type GaN layer on a sapphire substrate in this order, thereafter forming a p-type contact layer on the p-type GaN layer with a p-type contact metal such as Ni/Au, forming a DBR layer made of indium tin oxide (ITO) thereon and thereafter forming a support substrate by plating. In this structure, however, the light extraction efficiency is not sufficiently improved due to large light absorption by the contact metal such as Ni/Au. Further, while light absorption by ITO is negligible when the thickness of the DBR layer is about 300 nm, the light extraction efficiency may be insufficiently improved due to large light absorption if a thick film such as a multilayer film is prepared from ITO in order to form the DBR layer. While this document discloses no specific method of preparing the DBR layer from ITO, it is difficult to increase refractive index difference if a low refractive index layer is made of a material identical to that for a high refractive index layer, and the reflectance of the DBR layer made of ITO cannot be increased as a result.
Japanese Patent Laying-Open No. 2003-234542 discloses a nitride-based resonator semiconductor structure including a DBR, having a dielectric layer obtained by alternately stacking an SiO2 layer and a Ta2O5 layer, formed on a p-type contact layer with a thickness of a quarter wavelength. A support substrate is mounted on the DBR, a growth substrate is removed, an n-type layer and an active layer are thereafter removed by dry etching to expose the p-type layer, and a p-type electrode is formed on the exposed p-type layer. In the semiconductor structure described in this document, the dielectric layer having high reflectance is directly formed on the overall surface of the p-type contact layer, and hence a surface of the p-type contact layer opposite to the dielectric layer must be partially exposed for forming the p-type electrode thereon. However, a p-type nitride semiconductor layer has extremely high resistance, as sufficiently known in this technical field. Therefore, a current cannot laterally diffuse from the portion provided with the electrode. Even if the current slightly diffuses, resistance is extremely increased. Further, two electrodes must be formed on one side in this structure, to result in problems identical to the above.
A vertical resonator surface emission laser device disclosed in Japanese Patent Laying-Open No. 2004-119831 includes a DBR, consisting of a quarter-wavelength multilayer semiconductor structure of Si-doped n-type AlAs/AlGaAs, formed on a semiconductor substrate made of n-type GaAs. In order to implement a DBR having a structure similar to that of this DBR in a nitride semiconductor light-emitting device, a GaN substrate or an SiN substrate is generally employed as a conductive substrate. However, both of GaN and SiN are extremely high-priced and not suitable for a low-priced LED. In order to form the DBR by a multilayer structure of GaN and AlGaN formable by epitaxy on the GaN or SiN substrate, the layers must be extremely multicyclically grown due to small refractive index difference, and it is difficult to construct the DBR due to cracking or the like. In addition, the quality of an active layer grown on such a DBR is so inferior that internal quantum efficiency is reduced.
National Patent Publication Gazette No. 2008-506259 discloses a technique of forming a DBR consisting of conductive ZnSSe and an Mgs/ZnCdSe superlattice on a conductive GaAs substrate and welding the DBR to a nitride semiconductor layer provided on a sapphire substrate by wafer bonding. When the welding is performed without employing an adhesive, however, the surfaces of both wafers must indispensably be planar, and the wafer bonding cannot be performed if the surface of either wafer is slightly irregularized. In the case of an actual nitride semiconductor, waste or the like coming off a reactor frequently adheres to the surface of an epiwafer during epitaxy, and it is difficult to completely remove waste from the wafer.
Japanese Patent Laying-Open No. 2006-054420 discloses a light-emitting device having no vertical electrode structure but including a mesh DBR reflecting layer formed on a p layer of a flip chip light-emitting device and a contact electrode formed on a portion not provided with the mesh DBR reflecting layer. The mesh DBR reflecting layer is made of a nitride semiconductor. However, it is difficult to prepare a DBR from a nitride semiconductor as hereinabove described, and even if such a DBR can be prepared, no current is injected into a DBR region due to high resistance, and an injection area is reduced. Consequently, the current density is increased to reduce luminous efficiency. If the contact electrode portion has low reflectance, light extraction efficiency is also reduced. While this document lists Ag, Ni, Al, Ph, Pd, Ir, Ru, Mg, Zn, Pt, Au etc. as exemplary materials for an ohmic contact layer, these materials have low reflectance except Ag and Al. Al having high reflectance cannot be brought into ohmic contact with a p-type semiconductor layer, but increases resistance. If Ag is employed as the material for the ohmic contact layer, electromigration takes place to cause a short circuit upon migration to an n side, and hence this structure is extremely problematic in reliability.
As hereinabove described, there has hitherto been implemented no nitride semiconductor light-emitting device excellent in internal quantum efficiency, light extraction efficiency and driving voltage as well as in mass productivity.